Tests and Results
4.2 Battery Temperature Test
After smoothing and averaging the data a charger efficiency of around 83% was found.
When measuring the efficiency with multimeters the result was 83.6%. These results agree well with each other. Later on some of the passive components around the charger was changed. More specifically, the transient voltage suppressor diode was changed to another with smaller leakage current and the capacitor on the charger input and output was changed from tantalum to ceramic. This is recommended in the charger datasheet and the result was an increase in efficiency to 85%. See appendix D for plots of the efficiency tests.
4.1.5 Angle of Incidence
Depending on how well the ADCS works, the satellite will rotate during orbit. We cannot expect to have a fixed angle towards the Sun while orbiting. Therefore, it is important to see how quickly the charger reacts to changes in the angle of incidence. In figure 4.3 we see that the charger output follows the input almost instantaneous. We do not know how fast the satellite will be rotating but hopefully not more than a few hertz.
The angle of incidence changes were done by hand force and we can see that the charger reacts quickly to changes up to around 1 Hz. We can see that 45◦ angle of incidence is only lowering the PV-cell voltage by approx. 50 mV. When the angle of incidence is 90◦ the PV-cell voltage is lowered by approx. 500 mV.
4.2 Battery Temperature Test
Temperature is an important parameter when it comes to battery performance, both battery voltage and usable capacity. Our goal is to keep the battery voltage above 2.8 V for the greater part of the discharge curve. From the battery datasheet we find the discharge characteristics at 25◦, 0◦ and -20◦C. We find that somewhere between 0◦ and -20◦C the battery voltage quickly becomes too low for our application. It is important to find out at what temperature the voltage becomes too low, because this will tell us when the battery heating system should be activated. The battery was characterized when exposed to temperatures of 23◦, 0◦, -5◦, -10◦, -15◦ and -20◦C.
4.2.1 Test Setup
To reduce the charge/discharge time, a single battery cell was used in this test. The battery was charged to 95% with a TP610C charger and put the back panel and battery
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Current from charger [A]
0 2 4 6 8 10 12 14 16 18
(4.3a) Incident angle changing between 0 - 45◦.
0 2 4 6 8 10 12 14 16
Current from charger [A]
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(4.3b) Incident angle changing between 0 - 90◦.
Figure 4.3: Changing angle of incidence by hand force. We see how the angle of incidence affects the PV-cell voltage and current. We can see that the SPV1040 charger is quickly adjusting to changes in input power.
module into the test chamber. A temperature test chamber from Associated Testing Laboratories Inc. was used. This test chamber is old and has problems of maintaining stable temperature, especially at low temperatures. Slow variations of 1◦ to 2◦C around our wanted test temperature was experienced. This variance is acceptable, because it is not critical to find the exact temperature range for the battery heater operation.
The possibility of changing the TCS enable/disable threshold while deployed in space has been implemented. If the optimum thresholds is missed, the thresholds could be adjusted later. In addition, it is interesting to see how the battery reacts to these small temperature variations.
The temperature was stabilized and a discharge load of 3.3 Ω was connected. This results in a discharge current of approx. 1 A, at least before the battery voltage starts to decrease. The reason for choosing this load is that 1 A is the highest expected load current on the satellite, which occurs during radio transmission. The battery voltage and temperature was logged during discharge and the battery was discharged until reaching 2.5 V. INA226 was used for the voltage measurement and the TMP175 located on the battery module was used to log the temperatures. All temperature plots are found in appendix D. We remind you that the TMP175 sensor data has a typical accuracy of
±0.5◦C.
4.2.2 Test Results
In figure 4.4 we see the battery discharge characteristics for all the temperatures mea-sured. We see that the lower the temperature gets, the battery voltage and capacity is increasingly dependent on the temperature. For -22◦C and -15◦C we can see the small temperature changes in the test chamber directly affecting the battery voltage up to 50 mV. From the figure it seems like the battery capacity is lower at 23◦C compared to 0◦C but this is due to higher battery voltage at 23◦C resulting in a larger discharge current.
These measurements are worst-case measurements, because they were done with the heaviest expected load current. During normal operation, this load current will only occur in short transients. It is important that the satellite can withstand the worst-case current consumption without temporary lowering the battery voltage below the battery warning threshold.
The battery pack is oversized, which means the battery pack will only be discharged 5-10% during normal operation. Thus it is most interesting to see the first 5-10 minutes of the discharge curve, but in case the extra battery capacity is suddenly needed, we need to assure high enough battery voltage for the greater part of the discharge curve.
From these results we can see that down to -5◦C, we have a battery voltage well above 2.8 V for most part of the discharge curve. When reaching -10◦C, the battery voltage starts to decrease below 3.0 V significantly earlier. This tell us that the battery temperature should be kept above -10◦C. It is suggested that the TCS is enabled at -7◦C and disabled at -5◦C. This should ensure battery voltage above 2.8 V for most part of the discharge curve.
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Time [minutes]
Battery Voltage [mV]
avg t = −22oC avg t = −15oC avg t = −10oC avg t = −5oC avg t = 0oC avg t = 23oC Load connected
Figure 4.4: Battery discharge characteristics vs. temperature. Battery discharged into 3.3 Ω, making the discharge current dependent on the voltage. The temperature was slowly changing 1◦ to 2◦ around the stated temperature.